Squashed warnings and general clean-up

A number of things:
 * The Overlay component wasn't being used for anything but a debug stub, so I removed it.
 * A previous commit which added 1.10 compat caused GCC to complain about warnings due to string literals being returned by reference. I fixed this by adding a constant to the ParticleUniverseTypes.h header.
 * Moved StringStream to ParticleUniverseTypes.h.
 * For consistency - renamed the "cmake" folder to "CMake" as I'm pedantic. :P